Y-Chromosome Determination
Over the years a number of methods have been developed for making a determination either directly or indirectly of the presence of the Y chromosome in a particular tissue sample.
Barr Body Test
The barr body test is an indirect method for determining that an XY karyology may be present in a tissue sample. Buckle smear and other tissue samples dyed with Giemsa reagent stain reveal the presence of any X chromosomes beyond one copy per cell. These chromosomes appear as barr bodies within the cell nucleus.
Unfortunately, the buckle smear method is not always accurate in predicting the presence of a Y chromosome. For instance, an XO genotype would give the same results as an XY genotype. Also, at various stages in the cell growth and division cycle or in certain tissue samples, barr bodies fail to become visible. Therefore, barr body testing is generally limited to non-critical screening assays.
Because of the limitations of barr body analysis, this method is not appropriate for use in invasively sampled fetal cells, such as chorionic villi or amniocentesis samples. The invasive sampling techniques used are expensive and not without risks. Therefore, multiple sampling is generally unacceptable, as are delays in re-diagnosis which can occur when test results are equivocal. Even a low level of false diagnosis is unacceptable in situations where prenatal gender would be the basis of pregnancy termination.
Karyology
With the advent of tissue culture techniques, direct karyology of cells became possible. However, there are several limitations to this technique. The sample tissue must be in an active growth phase during analysis to be useful in karyology. This is because the sample cells must be dividing for the chromosomes to be arrested in their condensed, visible metaphase stage.
In the case of mammalian tissue sample growth systems such as are required in conventional human prenatal diagnosis, karyology techniques have proven to be time consuming and expensive. Mammalian cell lines are highly fastidious in their culturing parameters and cell media requirements. Further, any contamination of mammalian tissue culture material can result in complete failure of growth of the cells.